The present invention relates to a digital switching system, and more particularly to a variable rate TDM switching system for switching variable rate data traffic such as video traffic.
As known digital switching systems, a TDM (Time division Multiplexing) switch and an ATM (Asynchronous Transfer Mode) switch are broadly used.
The S-Switch (Space-switch) is the typical TDM switch. In the S-switch, data for a plurality of calls are time division multiplexed, and are transmitted in a TDM frame as a unit. The TDM frame is comprised of time slots having a length of about 1 octet, and the TDM frame period is about 125 xcexcs. In the TDM frame, the numbers of the time slots assigned to each of the calls and those positions are not changed during the establishment of the connection. FIG. 1 is a schematic view showing a principle of the S switch. The S switch comprises time-division gates provided at cross-points of input lines and output lines arranged in a lattice form, and a control memory for controlling the time-division gates. The S switch switches the TDM frames for multiplexing between the input lines and the output lines by switching the time-division gates at high speed, with a time slot as a unit. Each of the time slots in the input TDM frame is moved to the time slot in the output TDM frame on the output line of destination through the time-division gate specified by the control memory. Then, the temporal position of the time slot is kept so that the temporal position on the input line is identical with the temporal position on the output line. The control memory is provided at every output line and store schedules in which the time-division gates are opened to pass the time slot.
The TDM switch has an advantage that once the connection is setup, a delay assurance and a loss rate assurance can be guaranteed without performing a special control such as a flow control, a vergence control, etc. The TDM switch is now mainly used for a speech communication imposed with a strict delay assurance.
However, the TDM switch has the following problems. One problem is that a maximum rate of the TDM bus or a maximum rate of a RAM restricts increasing the capacity of the TDM switch. Furthermore, as the number of input lines increases, establishing synchronization of the input time slots becomes more difficult so that it is difficult to increase the capacity by more than about 10 Gbps.
Another problem is that it is difficult to handle multiple rate data traffic. Although the TDM switch provides means for switching efficiently same rate data traffic, the TDM switch cannot operate efficiently when there is traffic requiring various rates. Thus, it is difficult to handle multi-rate traffic because delay of the time slots is increased.
A further problem is that it is difficult to handle variable rate data traffic, since in the current TDM switching networks, in order to handle the variable rate traffic, the time slots must be required at the maximum rate of the traffic, and thus the transmission efficiency is reduced significantly. For example, in addition to the speech traffic, video traffic is also imposed with strict delay assurance. An MPEG2 is a typical video coding system. Coding due to the MPEG2 produces usually variable rate traffic. FIG. 3 is a graph representing a distribution of frame sizes of the MPEG2 stream. Assuming that an average available band of the variable rate traffic is about 1 Mbps and a maximum rate of the same band is about 5 Mbps, in the known TDM switching system, the connection must be set to about 5 Mbps as the maximum available band. However, because the average available band is about 1 Mbps, the traffic band of about 4 Mbps is wasted.
A cross-point buffer switch is the typical ATM switching system. FIG. 2 is a schematic view showing a principle of the cross-point buffer switch. The cross-point buffer switch comprises buffers each provided at cross-points between input lines and output lines arranged in a lattice form. Address filters are each followed by the buffers. Output control circuits are each connected to the output lines. In the cross-point buffer switch, data for a plurality of calls are time division multiplexed, and are transmitted in an ATM cell as a unit. The ATM cell has a length of 53 bytes, and comprises a header having a length of 5 bytes and a payload having a length of 48 bytes. Now, using the buffer Bij connected to the input port Ii and the output port Oj the operation of the cross-point buffer switch is described. The address filter Fij checks the header of the ATM cell inputted from the input line Ii and stores only the ATM cells destined for the output line Oj into the buffer Bij. When the cells to be sent are being stored in the buffer Bij, the buffer Bij requests permission to send the cells to the output control circuit Cj. The output control circuit Cj requires the buffer Bij to send the ATM cells in consideration of all requests of buffers related to output line Oj.
In the known ATM switch, because of the subdivision of the stream into the small ATM cells, an overhead caused by a cell assemble/disassemble and addition of the header becomes very large, and the switching speed becomes unnecessarily high. The known ATM switch also has a drawback that because the header is added to every ATM cell, high speed header processing is required. Moreover, for example, in the case of video traffic, when one ATM cell is lost, the quality of the picture can be significantly degraded. Furthermore, because the ATM switch cannot distinguish the types of data contained in the ATM cells, active processing is very difficult at the switch level. Therefore, controls in upper layers can be complicated.
It is an object of the present invention to avoid the problems of the above conventional switching system.
It is an object of the present invention to provide a variable rate TDM switching system suitable for handling efficiently the traffic imposed with strict delay assurance, particularly video traffic.
According to the invention, there is provided a method of operating a variable rate TDM switching system comprising the steps of:
arranging a plurality of incoming lines and a plurality of sending lines in lattice form;
providing time-division gates at cross-points of the incoming lines and the sending lines;
connecting time-division temporal switches having a function of transposing the order of time slots in input TDM frames to forward stage of respective incoming lines of the time-division space switches for performing the switching between the incoming lines and the sending lines with time slot unit while holding the multiplexing, by switching these time-division gates;
connecting a scheduler for concentratedly controlling the time-division gates and the time-division temporal switches to these gates and time switches;
transmitting the headers of the TDM frames to the scheduler in case of inputting the header of TDM frame, and deciding the temporal switching schedule by the scheduler as to the transposition of the order of time slots in, the time-division temporal switch, so as to disperse time slots of the same destination temporally;
deciding the spatial switching schedule as to the on off of the time-division gates in the time-division temporal switches so as to appear only one time slot of the destination on the same sending line at the same timing;
inputting the TDM frames into the time-division temporal switches with temporal switching by the time-division temporal switch in accordance with the temporal switching schedule; and
switching respective time slots into the sending lines of respective destinations.
In the variable rate TDM switch according to the invention, because the frame header contains the information about the positions of the time slots in the TDM frame, in the variable rate traffic, the connection does not need to be set at the maximum rate. If a too large amount of data is inputted into the switch, a plurality of TDM time slots can be assigned to transmit. Therefore, the variable rate traffic can be switched efficiently.
The known ATM switch switches the relatively small ATM cells, whereas the variable rate TDM switch according to the invention switches the relatively large TDM frame. Hence, compared to the known ATM switch, the switching delay increases with a low load. However, in the known ATM switch, the buffering delay increases with a high load. Whereas, in the variable rate TDM switch according to the invention, because the time slots destined for the same output line are temporally dispersed by the variable rate time switches, and then inputted to the variable rate space switch, the buffers are not required for the variable rate space switch. Therefore the delay characteristic is improved drastically. Moreover, in the known ATM switch, because the header is added to every ATM cell, high speed header processing is required. In the variable rate TDM switch on the other hand, because all information about the time slots in the TDM frame is contained in the TDM frame header, the header processing is easier than the known ATM switch.
According to the invention, there is provided a method of operating a variable rate TDM switching system comprising the steps of:
arranging a plurality of incoming lines and a plurality of sending lines in lattice form;
providing time-division gates at cross-points of the incoming lines and the sending lines;
connecting time-division time switches having a function of transposing the order of time slots in input TDM frames to forward stage of respective incoming lines of the time-division space switches for performing the switching between the incoming lines and the sending lines with time slot unit while holding the multiplexing, by switching these time-division gates,
connecting a scheduler for concentratedly controlling the time-division gates and the time-division temporal switches to these gates and time switches;
connecting contents analyzer for informing information from the TDM frame to the scheduler;
transmitting the headers of the TDM frames to the contents analyzer in case of inputting the header of TDM frame,
informing information as to the respective time slots in the TDM frame to the scheduler from the contents of the header of the TDM frame by the contents analyzer;
deciding the temporal switching schedule as to the transposition of the order of time slots in the time-division temporal switch in accordance with the information, so as to disperse time slots of the same destination temporally;
deciding the spatial switching schedule as to the on off of the time-division gates in the time-division temporal switches so as to appear only one time slot of the destination on the same sending line at the same timing;
inputting the TDM frames into the time-division spatial switches with temporal switching by the time-division temporal switch in accordance with the temporal switching schedule; and
switching respective time slots into the sending lines of respective destinations in accordance with the spatial switching schedule by the time-division spatial switch.
In this way, the switching schedule can be determined with considering the types of data contained in the time slots. For example, in the case of the MPEG2, the MPEG2 stream contains I picture, P picture and B picture. If the B picture is lost, the quality of the image is less degraded, whereas if the I picture is lost, the quality of the image is much degraded. Thus, the I picture should be given a higher priority than that given to the B picture. In the variable rate TDM switching system according to the invention, the contents analyzer can distinguish the types of the picture. By assigning the priority levels depending on the types of the picture to the time slots, the scheduler can determine in order the switching schedule of the priority levels.
In the variable rate TDM switching system according to the invention, the time slots stored in the variable rate time switches must be rearranged so that there is only one time slot destined for the same output line at the same time, because there is no buffer in the variable rate space switch. The various switching schedules may be used for this purpose. Therefore, in a preferable embodiment of the variable rate TDM switching system according to the invention, the time-division spatial switch decides the spatial switching schedule in such a manner that the time slots of different incoming lines are assigned in turn to the respective TDM frames of the sending lines.
In another preferable embodiment of the variable rate TDM switching system according to the invention, the time-division spatial switch decides the spatial switching schedule in such a manner that the number of time slots to be used are counted every a pair of the sending line and the incoming line, thereby selecting the time slots in order of large number.
In further preferable embodiment of the variable rate TDM switching system according to the invention, the time slot length of the TDM frame is 16 kbit and the frame period of the TDM frame is a divisor of a video frame period of an input video signal. In this way, in the video traffic, the video frame period can be corresponded to the TDM frame period.